1. Field of the Invention
The present invention relates to a cordierite honeycomb ceramic having a low thermal expansion coefficient and excellent heat resistivity, which is especially used for a catalyst carrier for an exhaust gas from automobiles and a filter for trapping soot included in an exhaust gas from a diesel engine, and a method of producing the same.
2. Related Art Statement
Cordierite honeycomb ceramics have an excellent heat resistivity, and thus they are used for a catalyst carrier for an exhaust gas from automobiles and a filter for trapping soot included in an exhaust gas from a diesel engine. In these fields, a high thermal shock resistivity is also required in addition to an excellent heat resistivity. If a thermal shock resistivity is low, various problems, such as crack generation and so on, occur due to rapid heating, a rapid cooling and a temperature difference in a body of cordierite honeycomb ceramics. Therefore, the life of the cordierite honeycomb ceramic is shortened. In order to make a thermal shock resistivity high, it is necessary to make a thermal expansion coefficient of cordierite honeycomb ceramics low. Heretofore, various experiments are performed for making a thermal expansion coefficient of cordierite honeycomb ceramics low.
Japanese Patent Laid-open Publication No. 64-3067 (JP-A-64-3067) discloses a method of making a thermal expansion coefficient of cordierite honeycomb ceramics low by controlling a particle size of raw materials for cordierite generation such as talc, kaolin and silica so as to improve thermal shock resistivity. However, in this method, it is necessary to limit porosity to 30-42% so as to maintain such an excellent thermal shock resistivity, and thus a filter made of cordierite honeycomb ceramics according to this method is not preferably used as a filter for trapping soots included in an exhaust gas from a diesel engine.
Moreover, Japanese Patent Laid-open Publication No. 49-129704(JP-A-49-129704) discloses a method such that a body of cordierite ceramics is brought into contact with an acid solution so as to improve thermal shock resistivity of the body of cordierite ceramics. In the body of cordierite ceramics treated by an acid solution, thermal shock resistivity is temporarily improved, since a thermal expansion coefficient is lowered. However, if the body is maintained for a long time in a high temperature condition such as a filter for trapping soot included in an exhaust gas from a diesel engine, the thermal expansion coefficient is increased, and thus an effect of improving thermal shock resistivity disappears.
Generally, in order to make a thermal expansion coefficient low on producing cordierite honeycomb ceramics, amounts of impurity components in raw materials are lowered. For example, if an amount of CaO component included in raw materials for cordierite generation is increased, a thermal expansion coefficient of cordierite honeycomb ceramics becomes high. Moreover, as for a ferric component and the like, it is generally understood that it is better that a ferric component and the like is not included in cordierite honeycomb ceramics. This is because a ferric component is liable to be dissolved in a solid state in chlorite, which exists in talc as an impurity component. That is to say, a chlorite crystal having a ferric solid solution becomes a glass phase by firing, and becomes a cause of increased thermal expansion coefficient of cordierite honeycomb ceramics.
Japanese Patent Laid-open Publication No. 61-178038 (JP-A-61-178038) discloses a catalyst carrier in which an Fe.sub.2 O.sub.3 component is mixed in cordierite honeycomb ceramics and a coloring of the catalyst carrier is performed while maintaining a thermal expansion coefficient under 1.6.times.10.sup.-6 /.degree. C. However, in this case, the Fe.sub.2 O.sub.3 component functions only to color the catalyst carrier, and a thermal shock resistivity is affected since a thermal expansion coefficient is high.